Tool Wear, Tool life & Machinability

Tool Life

Useful cutting life of tool expressed in time

Time period measured from start of cut to failure

of the tool

Time period b/w two consecutive resharpenings

or replacements.

Ways of measuring tool life

No. of pieces of work machined

Total volume of material removed

Total length of cut.

Limiting value of surface finish

Increase in cutting forces

Dimensional accuracy

Overheating and fuming

Presence of chatter

Modes of tool failure

1. Temperature failure

a. Plastic deformation of CE due to high temp

b. Cracking at the CE due to thermal stresses.

2. Rupture of the tool point

a. Chipping of tool edge due to mechanical impact

b. Crumbling of CE due to BUE

3. Gradual wear at tool point

a. Flank wear

b. Crater wear

Tool wear

Tool wear causes the tool to lose its original

shape- ineffective cutting

Tool needs to be resharpened

Causes of Tool Wear

1. Attrition wear

2. Diffusion wear

3. Abrasive wear

4. Electrochemical wear

5. Chemical wear

6. Plastic deformation

7. Thermal cracking

Attrition wear

At low cutting speeds

Flow of material past cutting edge is irregular and

less stream lined

BUE formed and discontinuous contact with the

tool

Fragments of tool are torn from the tool surface

intermittently

High

Slow and interrupted cutting

Presence of vibrations

Found in carbide tools at low cutting speeds

Diffusion wear

Diffusion of metal & carbon atoms from the tool

surface into the w/p & chip.

Due to

High temp

High pressure

Rapid flow of chip & w/p past the tool

Diffusion rate depends on the metallurgical

relationship

Significant in carbide tools.

Abrasive wear

Due to

Presence of hard materials in w/p material.

Strain hardening induced in the chip & w/p due to

plastic deformation.

Contributes to flank wear

Effect can be reduced by fine grain size of the

tool & lower percentage of cobalt

Electrochemical wear

When ions are passed b/w tool & w/p

Oxidation of the tool surface

Break down of tool material @ chip tool interface

Chemical wear

Interaction b/w tool and work material

Plastics with carbide tools

Cutting fluid

Plastic Deformation

When high compressive stresses acts on tool

rake face- tool deformed downways – reduces

relief angle

Modifies tool geometry and accelerates other

wear processes

Thermal cracking

Due to cyclic thermal stresses at cutting edge

Comb cracks

Transverse cracks

Chipping of tool

Geometry of tool wear

Flank wear (edge wear)

Crater wear (face wear)

Flank Wear

Tool slides over the surface of the work piece and

friction is developed

Due to Friction and abrasion.

Adhesion b/w work piece & tool- BUE

Starts at CE and starts widening along the

clearance face

Independent of cutting conditions and tool / work

piece materials

Brittle and discontinuous chip

Increases as speed is increased.

Primary stage rapid

wear due to very high

stress at tool point

Wear rate is more or

less linear in the

secondary stage

Tertiary stage wear

rate increases rapidly

resulting in

catastrophic failure.

Crater wear

Direct contact of tool and w/p

Forms cavity

Ductile materials – continuous chips

Initiates rapid rupture near to nose

Leads to

weakening of the tool

Increase in cutting temp

Cutting forces & friction

Measurement of tool life Time for Total destruction in case of HSS or time

to produce 0.75 mm wear for carbide tools

Tool life expressed by Taylor’s eqn

VTb = C V = cutting speed in cm/min

T= tool life in min

b= const= 0.1 for HSS

C= 50 for HSS

Cemeted carbide : b=0.125, C=100

Tool life expressed in volume of metal removed L = TVfd

Measurement of tool life

Diamond indentor technique

Radioactive techniques

Test at elevated cutting speeds

Facing tests

Test with low wear criterion

Factors affecting tool life

1. Cutting speed

2. Physical properties of w/p

3. Area of cut

4. Ratio of feed to depth of cut

5. Shape and angles of tool

6. Tool material and its heat treatment

7. Nature and quantity of coolants

8. Rigidity of tool and w\p

Machinability

Machinability is defined in terms of:

1. Surface finish and surface integrity

2. Tool life

3. Force and power required

4. The level of difficulty in chip control

Good machinability indicates good surface finish and

surface integrity, a long tool life, and low force and power

requirements

Machinability ratings (indexes) are available for each type

of material and its condition

Factors affecting machinability of

metals

1. Material of w/p- hardness, tensile properties, strain hardenability

2. Tool material.

3. Size and shape of the tool.

4. Type of machining operation.

5. Size, shape and velocity of cut.

6. Type and quality of machine used

7. Quality of lubricant used in machining

8. Friction b/w chip & tool

9. Shearing strength of w/p material

Evaluation of machinability- factors

Tool life

Form and size of chip and shear angle.

Cutting forces and power consumption

Surface finish

Cutting temperature

MRR per tool grind

Rate of cutting under standard force

Dimensional accuracy

Evaluation of machinability

Machinability decreases with increase in tensile

strength and hardness

Machinability of a material is assessed by any of

the following.

Tool life

Limiting MRR at which the material can be

machined for standard short tool life.

Cutting force

Surface finish

Chip shape

Relative machinability Mg alloys

Bearing bronze

Al alloys

Zn alloys

Free cutting sheet brass

Gun metal

Silicon bronze, Mn bronze

S.G Cast iron

Malleable cast iron

Gray CI

Free cutting steel

Sulphur bearing steel

Cu-Al alloys

Low carbon steels

Nickel

Low alloy steels

Wrought iron

HSS

18-8 SS

Monel

White CI

Stellite

Sintered carbides

Machinability index

Machinability index= Vt/Vs x100

Vt – cutting speed of metal for 1 min tool life

Vs – cutting speed of standard free cutting steel

for 1 min tool life.

Material MI

SS 25

Low carbon steel 55-65

Cu 70

Red brass 180

Al alloys 300-1500

Mg alloys 500-2000

Machinability:

Machinability of Ferrous Metals

Steels

If a carbon steel is too ductile, chip formation can produce built-up

edge, leading to poor surface finish

If too hard, it can cause abrasive wear of the tool because of the

presence of carbides in the steel

In leaded steels, a high percentage of lead solidifies at the tips of

manganese sulfide inclusions

Calcium-deoxidized steels contain oxide flakes of calcium

silicates (CaSO) that reduce the strength of the secondary shear

zone and decrease tool–chip interface friction and wear

Machinability:

Machinability of Ferrous Metals

Effects of Various Elements in Steels

Presence of aluminum and silicon is harmful, as it combine with

oxygen to form aluminum oxide and silicates, which are hard and

abrasive

Thus tool wear increases and machinability reduce

Stainless Steels

Austenitic (300 series) steels are difficult to machine

Ferritic stainless steels (also 300 series) have good machinability

Martensitic (400 series) steels are abrasive

Machinability:

Machinability of Nonferrous Metals

Aluminum is very easy to machine

Beryllium requires machining in a controlled environment

Cobalt-based alloys require sharp, abrasion-resistant tool

materials and low feeds and speeds

Copper can be difficult to machine because of builtup edge

formation

Magnesium is very easy to machine, with good surface finish and

prolonged tool life

Titanium and its alloys have very poor thermal conductivity

Tungsten is brittle, strong, and very abrasive

Cutting fluids

Decreasing power requirement

Increasing heat dissipation

Neat oils+ extreme pressure additives

Water emulsions